Polymeric casting materials have gained widespread commercial acceptance during the past decade. As compared to Plaster of Paris casts, the polymeric or so-called "synthetic" casting materials have advantages of being lightweight and strong. Synthetic casting materials are also porous in the hardened state so that the cast is breathable. Typically, orthopedic synthetic casts are made from a curable resin-impregnated narrow fabric or casting tape. The fabric, which is preferably knit, can be formed of glass fibers, and/or synthetic fibers, such as polyester, nylon, polyolefins, and the like.
Water hardenable polyurethane prepolymers disclosed in Yoon, U.S. Pat. No. 4,433,680, constitute a particularly desirable hardenable resin for use in synthetic casting materials. These casting materials employ a water activatable polyurethane prepolymer which contains a dimorpholinodiethylether catalyst. These compositions have a long shelf life during which the polyurethane polymer remains in a liquid state. When the bandage is to be used, it is removed from a sealed package and placed in water for a few seconds. It is removed from the water and applied to the patient, usually over a tubular, knitted fabric and a padding. The bandage will set rapidly to a condition where it is capable of immobilizing a fracture.
The properties and functionality exhibited by the finished cast result both from the fabric and from the resin employed in constructing the casting tape. Ideally, the final cast has both a high degree of strength and a high porosity, resulting in a breathable cast. It is also important that the final cast conforms uniformly to the irregular surfaces of the patient's body. This is necessary in order that the damaged portion of the body be fully supported during healing. In addition high conformability minimizes the formation of wrinkles and puckers which can result in pressure areas in the cast that can cause patient discomfort and interfere with healing. Conformability is also important for ease of application. Surgeons and casting technicians can provide more cost efficient services when applying conformable fabrics which easily adapt to body contours without generating tucks and bulges which, as discussed previously are objectionable both from a patient comfort and healing standpoint, and therefore must be removed or minimized during the wrapping process.
High modulus fibers, such as glass fibers, are widely used in synthetic casting materials due to their ability to impart high strength characteristics to the final casting bandage. In addition, the use of fibers having a high modulus or stiffness minimizes the tendency of the fabric to become compressed and flattened while being coated and wrapped. This in turn, preserves both the shape and the mesh structure of the fabric resulting in a porous cast with a high degree of breathability.
Although the high modulus fibers have a beneficial impact on cast strength, the high modulus fibers also provide stiffer fabrics which typically do not conform well to complex surfaces. In addition, the need for high strength dictates the use of relatively high density fabrics, which decreases still further the ability of the fabric to conform uniformly to irregular body surfaces.
The ability of a fabric to process well during a liquid resin coating operation is also important. Thus, the fabric must remain flat as it is fed to and passed between coating rollers which apply a resin coating to the fabric. Moreover, the resin distribution along the fabric must be uniform to ensure uniformity in the final cast.
Buese et al., U.S. Pat. No. 4,668,563 discloses high modulus casting tapes of improved conformability. These casting tapes contain a casting fabric formed of a combination of high modulus yarns and elastomeric yarns. The elastomeric yarns are incorporated into the fabric along the length direction to give the fabric an extensibility of between 40% and 200% in the length direction. Preferably, these fabrics are Raschel Warp Knit fabrics having the elastomeric yarns forming or distributed within the wale yarns. The elastomeric yarns are incorporated into the fabric during the knitting process under a predetermined amount of tension so that the finished fabric gathers or bunches to a moderate degree when it is released from the knitting machine. The resulting fabrics are impregnated with a hardenable prepolymer to provide a casting tape of substantially improved conformability and which has experienced wide-spread commercial success.
Despite the availability of casting tapes of improved conformability, various improvement in strength, cost and the like are still desirable. However, the design of improved casting tapes must be exercised within the limits imposed by various practical constraints. Specifically, improving the strength of a casting tape is typically accomplished by using more high modulus fibers per unit area of the tape. However, this in turn increases the cost of the fabric, increases the stiffness and decreases the conformability. Moreover, fabric processability must be retained in order that the fabric be commercially useful as a practical matter. Thus, the practical constraints involved in casting tape manufacture dictate a trade off between strength and cost, and between conformability and strength, while requiring a good processability.